This study of laminar non-premixed HC-air flames used an Oscillatory-input Opposed Jet Burner (OOJB) system developed from a previously well-characterized 7.2-mm Pyrex-nozzle OJB system. Over 600 dynamic Flame Strength (FS) measurements were obtained on unanchored (free-floating) laminar Counterflow Diffusion Flames (CFDF's). Flames were stabilized using plug inflows having steady-plus-sinusoidal axial velocities of varied magnitude, frequency, f, up to 1600 Hz, and phase angle from 0 (most data) to 360 degrees. Dynamic FS is defined as the maximum average air input velocity (U air , at nozzle exit) a CFDF can sustain before strain-induced extinction occurs due to prescribed oscillatory "peak-to-peak" velocity inputs superimposed on steady inputs.Initially, dynamic flame extinction data were obtained at low f, and were supported by 25-120 Hz HotWire cold-flow velocity data at nozzle exits. Later, expanded extinction data were supported by 4-1600 Hz Probe Microphone (PM) pk/pk P data at nozzle exits. The PM data were first obtained without flows, and later with cold stagnating flows, which better represent speaker-diaphragm dynamics during runs. The PM approach enabled characterizations of Dynamic Flame Weakening (DFW) of CFDF's from 8 to 1600 Hz. DFW was defined as % decrease in FS per Pascal of pk/pk P oscillation, namely, DFW = -100 d(U air / U air,0Hz ) / d(pkpk P). The linear normalization with respect to acoustic pressure magnitude (and steady state (SS) FS) led to a DFW unaffected by strong internal resonances.For the C 2 H 4 /N 2 -air system, from 8 to 20 Hz, DFW is constant at 8.52 + 0.20 (% weakening)/Pa. This reflects a "quasi-steady flame response" to an acoustically induced dU air /dP. Also, it is surprisingly independent of C 2 H 4 /N 2 mole fraction due to normalization by SS FS. From 20 to ~ 150 Hz, the C 2 H 4 /N 2 -air flames weakened progressively less, with an inflection at ~ 70 Hz, and became asymptotically insensitive (DFW ~ 0) at ~ 300 Hz, which continued to 1600 Hz. The DFW of CH 4 -air flames followed a similar pattern, but showed much greater weakening than C 2 H 4 /N 2 -air flames; i.e., the quasi-steady DFW (8 to ~ 15 Hz) was 44.3 %/Pa, or ~ 5x larger, even though the 0 Hz (SS) FS was only 3.0 x smaller. The quasi-steady DFW's of C 3 H 8 -air and C 2 H 6 -air were intermediate at 34.8 and 20.9 %Pa, respectively. The DFW profiles of all four fuels, at various frequencies, correlated well but non-linearly with respective SS FS's. Notably, the DFW profile for C 3 H 8 -air fell more rapidly in the range > 15 to 60 Hz, compared with the 1-and 2-carbon fuels. This may indicate a shift in chemical kinetics, and/or O 2 transport to a flame that moved closer to the fuel-side.
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